WASHINGTON — The dispute over whether Boeing built an assembly plant in South Carolina in retaliation against its strike-prone Machinists union turned increasingly partisan Wednesday, when a Republican-controlled House committee announced it would hold a hearing to investigate whether federal regulators were forcing workers into unions.

The title of the June 17 hearing — "Unionization Through Regulation: The NLRB's Holding Pattern on Free Enterprise" — was a blunt reminder of the fierce Republican opposition to a recent National Labor Relations Board complaint challenging Boeing's decision.

And the hearing will be held not on Capitol Hill, but in North Charleston, S.C., the site of the 787 Dreamliner plant.

Rep. Darrell Issa of California, chairman of the Committee on Oversight and Government Reform, called for testimony from the top lawyer with the NLRB. The agency filed a complaint alleging that Boeing made threats to discourage lawful union activity and decided not to put the second assembly line for its 787 jetliner in Everett to punish Machinists workers for past strikes and to warn against future ones.

Issa's action represents the latest escalation of the fight between the NLRB, which now has a majority of Democratic appointees, Boeing and GOP elected officials.

Even before Issa's decision, 19 Senate Republicans asked President Obama to withdraw his nomination of the NLRB attorney, Acting General Counsel Lafe Solomon. In addition, Republican presidential candidate Newt Gingrich has urged the GOP to make its consent on raising the federal debt limit conditional on cutting off funding for the NLRB.

Pushing back, 10 Democratic members of the Senate Health, Education, Labor and Pensions Committee — including Sen. Patty Murray — wrote Solomon a letter telling him to ignore "political pressure or other extraneous factors" swirling around the Boeing case.

All that was prelude to next Tuesday's NLRB hearing in Seattle when an administrative-law judge will hear opposing arguments in the Boeing case for the first time.

Connie Kelliher, spokeswoman for International Association of Machinists Local 751 in Seattle, accused Issa of political interference. "Boeing and its supporters are trying to make this a political issue because they do not have a legal case," Kelliher said. "Boeing broke the law and Boeing is not above the law."

In filing the complaint against Boeing, Solomon noted that the Machinists went on strike against the company five times since 1977, most recently in 2008.

Boeing's chairman, vice presidents and numerous other executives, the complaint alleges, publicly referred to that history of labor strife in explaining the 2009 decision to build a final-assembly line outside of Washington state for the first time.

If Boeing is found at fault, Solomon proposed that it be required to operate the second 787 production line in Everett — regardless of what happens with the nonunion South Carolina plant. That facility is scheduled to open next month and deliver its first airplane in 2012.

South Carolina is a "right to work" state where individual employees can join unions voluntarily, but unions cannot force membership across entire work sites.

Peter Schaumber, a former Republican member of the NLRB's five-member board, contends that the Boeing case reflects organized labor's influence on the agency.

"This complaint is extreme," Schaumber said. "Any employer has the right to open a new facility anywhere."

Kelliher counters that Boeing has spread much of its jetliner production work around the globe without triggering union complaints of unfair labor practices. The difference this time, she contends, was that Boeing didn't conceal that its motive was to chill future strikes.

"It's not even a complicated legal argument," Kelliher said. "The line rightfully should have been" in Everett.

Charles Craver, a labor-law expert at George Washington University School of Law, said Boeing executives' comments on their face could constitute illegal coercive statements that threatened the workers' right to strike. So Craver believes Solomon's decision to file the complaint was prudent.

The legal challenge, however, would be connecting those statements directly to opening of the North Charleston plant. Boeing could plausibly argue, Craver said, that it actually moved to South Carolina because wages are lower or because of tax incentives.

Craver said one fact is incontestable.

Boeing executives should have "kept their mouths shut. I'm sure their labor lawyers were banging their heads."